Diffuse optical spectroscopy (DOS) using frequency modulated light has been employed to quantify in-vivo tissue constituents as well as the optical properties of in-vivo tissue. Diffuse optical spectroscopy may also be used to quantify chromophore concentration in biological material. Diffusion approximation to the equation of radiative transport provides a useful modeling framework for diffuse optical spectroscopy methods, and generally gives an accurate description of light propagation in thick tissues provided detected photons have undergone at least 10 scattering events before they reach the detector. For example, U.S. Pat. No. 5,424,843 and U.S. patent application Publication No. 20030023172, both of which are incorporated by reference as if set forth fully herein, describe frequency domain spectroscopy methods and devices used in analyzing light scattered from a sample. While the ‘843 patent and the ‘172 published application provide a general framework for recovering chromophore concentration and optical properties, neither describes a means for accurately recovering these quantities for superficial tissues or under conditions when the source-detector separation becomes smaller than that allowed by diffusion approximation.
Consequently, current diffuse optical spectroscopy methods are limited to situations in which the reduced scattering coefficient, μs′ is greater (by an order of magnitude) than the absorption coefficient, μa. In practical terms, this limits the DOS technique to source-detector separations of about 5 mm in most tissues (with interrogation depths of about 2.5 mm), wavelengths between 650-1000 nm, and modulation frequencies between 50 and 600 MHz.
As source-detector separation is reduced to distances smaller than 5 mm, the validity of diffusion approximation is reduced along with ability to accurately recover optical properties and chromophore concentrations in existing diffuse optical spectroscopy methods and devices. As the distance between the source and detector becomes smaller, the average number of scattering events that photons experience before detection is also reduced. Similarly as one moves to more highly absorbing spectral domains (e.g., wavelengths shorter than 650 nm and wavelengths longer than 1000 nm), a reduction in source-detector separation is necessary in order to collect light with a reasonable signal to noise (SNR) ratio. In each of these scenarios, however, a simple application of diffusion approximation-based modeling will yield inaccurate tissue optical properties and chromophore concentrations.
U.S. Pat. No. 6,678,541, which is incorporated by reference as if set forth fully herein, discloses an optical fiber probe and methods for measuring optical properties. This approach, however, requires a multi-fiber probe geometry in order to recover tissue optical properties under continuous illumination. The disadvantage of a multi-fiber probe geometry is that each fiber samples a slightly different volume of tissue so there is inherent inaccuracy in the method.
With respect to the problem of quantifying superficial chromophores and their optical properties, prior methods have solved this by primarily using multivariate calibration techniques such as the method of Partial Least Squares (PLS). In this method, signals are acquired from a set of samples that are representative of the sample of interest. The concentration of the analyte of interest must be known for each sample included in the calibration. By sampling many “reference” samples, an empirical model relating spectral shapes to analyte concentration can be developed. The problem with this approach is that the calibration samples have to be very similar to the target (unknown) sample set of interest. In addition, there has to be a way of recovering the true concentration of the analyte of interest in each of those samples, using a separate method, so that a correlative model can be developed.
There thus is a need for device and method which can perform reliable diffuse optical spectroscopy measurements where the source-detector distance is reduced (for example, less than 5 mm). Reducing the distance between the source and detector while still allowing for accurate quantification using diffusion approximation would advantageously allow smaller probe-type devices to be manufactured. In addition, smaller source-detector distances would permit diffuse optical spectroscopy analysis of superficial volumes in biological tissue. The analysis of superficial volumes using diffuse optical spectroscopy has applications for the quantitative characterization of epithelial malignant transformation in tissues which generally occurs at depths of a few tens of microns to a few hundred microns. The method and device would also be able to determine the optical properties and even quantify chromophore (e.g., glucose) concentrations of tissue components in-vivo at superficial depths (tissue depths for determining interstitial tissue glucose concentration/distribution range from a few tens of microns to a few hundreds of microns depending on body site probed). The method and device would have potential intravascular applications to characterize vulnerable plaques, sub-surface pools of lipids, and inflammatory changes occurring in vascular tissue. The method and device would have potential applications in the assessment of effectiveness of pharmaceutical and/or cosmetic formulations that may be used to alter the appearance or “quality” of skin. For example, the device is particularly amenable to measuring changes in superficial tissue hydration. The method and device would have potential applications in the in-situ characterization of skin surface preparations such as sunscreens. Finally, the method may be used in connection with non-biological samples such as, for example, quantifying chemical species in tablet formulations.